Many chromogenic phenomena are known in which a change in color or a change in light absorption results from some action or stimulus exerted on a system. The most common chromogenic phenomena are electrochromics, (EC), photochromics, (PC), and thermochromics, (TC). Many phenomena are also known in which optical changes, like light scattering or diffuse reflection changes, take place as a result of some action or stimulus exerted on a system.
Unfortunately, referring to these as chromic phenomena has led to a fair amount of confusion in the past. We prefer to distinguish light scattering systems from chromogenic systems by referring to the light scattering phenomena as a phototropic, thermotropic or electrotropic phenomena. This distinction and other distinctions are elaborated on below.
In general, and especially for the sake of the patent application, the terms used for an optical phenomena, should relate to the direct, primary action causing the phenomena. For example, modern day electrochromic systems generally involve electrochemical oxidation and reduction reactions. Thus an electrical process directly causes materials to change their light absorbing or light reflecting properties. Alternatively, electrical energy can also be used to generate heat or light and this heat or light, in turn, may be used to affect a thermochromic or a photochromic change. However, the indirect use of electricity should not make these electrochromic phenomena. For example, a thermochromic layer may increase in temperature and light absorption when in contact with a transparent conductive layer which is resistively heated by passing electricity through the transparent conductive layer. However, in accordance with the terminology used herein, this is still a thermochromic device and should not be called an electrochromic device. Also, just because an electric light produced UV radiation that caused a color change by a photochemical reaction, like the ring opening of a spirooxazine compound, that would not make such a procedure a demonstration of electrochromics.
A similar distinction should be made with a thermochromic layer that is responsive to sunlight as described in U.S. Pat. Nos. 6,084,702 and 6,446,402. The thermochromic layer may be heated by absorbing sunlight or being in contact with another layer that absorbs sunlight. Here sunlight exposure changes the color and/or the amount of light absorbed by the thermochromic layer. However, this is still a thermochromic phenomenon because a heat induced temperature change causes the chromogenic change and the same change takes place when the layer is heated by other means. The absorbed photons from the sun are only converted to heat and do not directly cause a photochromic change. Accordingly, the term photochromics should be reserved for systems in which the absorption of a photon directly causes a photochemical or photophysical reaction which gives a change in color or a change in the system's ability to absorb other photons.
In addition to chromogenic systems, there are a variety of systems with reversible changes in light scattering. The more widely studied light scattering systems include: (1) lower critical solution temperature, LCST, polymeric systems; (2) polymer dispersed liquid crystal, PDLC, systems; (3) polymer stabilizer cholesteric texture, PSCT, systems and (4) thermoscattering, TS, systems. Additional description of these and other light scattering phenomena may be found in U.S. Pat. No. 6,362,303. In the past, several of these phenomena have been called thermochromic and even electrochromic. From our standpoint these phenomena are neither thermochromic nor electrochromic since the word chroma relates to color and the intensity and quality of color. These are better termed thermotropic or electrotropic to help indicate the change in state that takes place.
Definitions rarely cover every eventuality, especially when it comes to borderline cases. Hence electrochemical systems that change from colorless and non-light scattering to specularly reflecting are still generally termed electrochromic because of the electrochemical nature of these processes. Also, some thermochromic systems involve changes between liquid and solid phases and could conceivably be called thermotropic systems. But these systems have dramatic changes in light absorption and are still termed thermochromic. On the other side, some reversible light scattering systems may have some spectral selectivity to the light scattering and hence give rise to some color appearance. Yet the primary change is between light scattering and non-light scattering states. Even the change in some systems from colorless and non-light scattering to a frosted, diffusely reflecting and white appearance might suggest a color change to the color white. However, we still term these tropic and not chromic changes.
In summary, systems without any substantial change in light scattering, that primarily involve a change in color, intensity of color or absorption of light, as well as those electrochemical and thermochemical phenomena that give a change in specular reflectance, are herein understood to be chromic or chromogenic phenomena. One of these chromic phenomena—thermochromics, as defined herein, is the subject of the present invention.
Many thermochromic materials and phenomena are known. These include reversible and irreversible changes in optical character. A well known thermochromic phenomena, for use with windows, involves metal oxide thin films. Most notably films of VO2, and doped versions thereof, are known to reversibly change their specular reflectance in the NIR with changes in temperature. Thermochromic processes with changes in light absorption are observed when heating causes: (1) an increase in the amount of ring opening of certain spiro compounds; (2) the dissociation of certain anions from certain triarylmethane dyes or (3) the dissociation of certain “dimeric” substances into highly absorbing “monomeric” free radicals. Thermochromic phenomena are also involved in phase change systems which change from highly absorbing to colorless or nearly colorless when certain pH indicators change their association with certain weak acids during a melting or solidification process.
Still other reversible thermochromic systems involve thermally induced changes in the way ligands associate with transition metal ions. The present application discloses particularly useful versions of these metal-ligand thermochromic systems and combinations of these systems with other thermochromic systems.